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9.9 Forensic Chemistry: 6. Spectroscopy and elements
Extracts from Chemistry Stage 6 Syllabus (Amended
October 2002). © Board of Studies, NSW.
[Edit: 9 Jul 09]
Prior learning: Preliminary module 8.2.3
describe the conditions under which atoms will emit light
- Atoms in their normal state do not emit light.
- If an atom is given extra energy by either heat or electricity, the electrons within the atom are excited into a higher energy level. When the electrons fall back down to their ground state (lowest energy level – atoms are normally in this state) the atom will emit light (energy).
The wavelength and frequency of light emitted is related to the energy released. The greater the energy, the shorter the wavelength and the greater the frequency.
Energy Released Electromagnetic Radiation Wavelength Frequency Small IR light Long Low Medium Visible light Medium Medium Large U.V. radiation Short High
identify that the emission of quanta of energy as electrons move to lower energy levels may be detected by humans as a specific colour
- White light is the combination of all colours of the spectrum. The spectrum ranges from 375nm (violet) to 740nm (red). The spectrum can be split into three basic sections Ultraviolet light (<375nm), white light (375-740nm) and Infra-red radiation (>740nm).
Electromagnetic spectrum
, Socrates, University of California, Berkeley, California, USA.
- Each colour corresponds to a wavelength. Red colour has a longer wavelength than blue colour. The wavelength is inversely proportional to the amount of energy released. This means that as the amount of energy released decreases the wavelength increases. Less energy is required to produce a red colour (longer wavelength) than blue colour.
explain why excited atoms in the gas phase emit or absorb only certain wavelengths of light
- Each individual excited atom usually emits only one wavelength. Not all atoms in a sample will absorb or be excited in exactly the same way and therefore excited electrons will travel to different energy levels (different distances from the natural ground state).
- Each element produces a different wavelength of light (colour) because each element has a unique energy level system (energy shell spacing). Therefore when an atom is excited, electrons will travel to different shells according to the element.
account for the fact that each element produces its signature line emission spectrum
discuss the use of line emission spectra to identify the presence of elements in chemicals
process and present information from secondary sources to analyse and identify individual elements present in a mixed emission spectrum and use available evidence to explain how such information can assist analysis of the origins of a mixture
The following information addresses the above three syllabus points at the same time.
- When light is analysed by spectroscopy it is separated into its component colours by passing the light beam through a prism or diffraction grating. The individual colours can be separated to form a series of individual lines by focussing the light through a narrow slit. The lines formed are representative of the wavelengths of colour formed by the emission of energy from the particular element.
- Since each element has a unique combination of colour wavelengths produced by the electrons release of energy, a specific series of lines are formed. The series of lines are called a spectrum. A spectrum formed by the emission of energy is called an emission spectrum.
- Emission spectra can be used to identify an unknown element or a mixture of elements.
- Use secondary sources, such as the web site below to identify the emission spectra of common elements, including sodium and hydrogen produced by gas discharge tubes. Present this information by drawing the spectra for each element to a correct scale. Use different coloured pens or pencils.
Spectra
- Using the spectra you have drawn analyse spectra produced by different mixtures to identify the individual elements present in the mixture. Process the information by comparing the individual spectral lines from elements and the mixture. Present the information by drawing the mixture spectrum and labeling each line with the compound responsible.
- Use the available evidence you have collected in the form of spectra for both elements and mixtures to explain how spectra from individual elements allow the identification of elements within a mixture.
Interactive experiment identifying the emission spectra
identify data, choose equipment, plan, and perform a first-hand investigation using flame tests and/or spectroscope analysis as appropriate to identify and gather first-hand information to describe the emission spectrum of a range of elements including Na and Hg
Background information
Flame tests can be performed by either using solid samples of metal compounds or metal compound solutions. Solid samples can be placed into the flame of a Bunsen burner using a paper clip held onto a cork. It is important that the paper clip is thoroughly cleaned between each sample. Solution samples can be spayed from a spray bottle directly into the flame. It is important to ensure that no sample enters the Bunsen burner as this will affect further results. A hand held spectroscope is used to observe the spectrum.
Spectroscope analysis requires the use of gas discharge tubes. Electrical energy is used to excite the atoms in the tube, producing a distinctive colour light for each element. A hand held spectroscope is used to observe the spectrum.
This experiment can be conducted either in the laboratory or by using a simulation on an appropriate web site such as below.
Identification of
emission spectra obtained from flame tests and gas
discharge tubes, Dr Walt Volland, Bellvue Community College, Spokane, Washington, USA.
- The choice of equipment for this investigation is dependent on the method used, i.e. flame test, gas discharge tube or web site simulation.
- The use of a web site simulation must also include an investigation plan.
- Use the web site to perform a first-hand investigation to identify spectra related to individual elements, resulting from specific flame tests and gas discharge tubes. Gather first-hand information by drawing to scale the spectra of individual elements including Na and Hg.
- A CD ROM disk can also be used to view the emission spectra emitted from yellow (Na) and white (H) street lights. The CD ROM disk should be held horizontally (label down) to the light.
The CD ROM spectroscope
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